A variety of multiple access communication systems and techniques have been developed for transferring information among a large number of system users, such as code division multiple access (CDMA) spread spectrum techniques. CDMA techniques in multiple access communication systems are disclosed in the teachings of U.S. Pat. No. 4,901,307, which issued Feb. 13, 1990 under the title xe2x80x9cSpread Spectrum Multiple Access Communication System Using Satellite Or Terrestrial Repeatersxe2x80x9d, and U.S. patent application Ser. No. 08/368,570, filed under the title xe2x80x9cMethod And Apparatus For Using Full Spectrum Transmitted Power In A Spread Spectrum Communication System For Tracking Individual Recipient Phase Time And Energy,xe2x80x9d which are both assigned to the assignee of the present invention, and incorporated herein by reference. These patents disclose communication systems in which communication signals are transferred through satellite repeaters and gateways, or terrestrial base stations (also referred to as cell-sites or cells).
In a typical spread-spectrum communication system, one or more preselected pseudorandom noise (PN) code sequences are used to modulate or xe2x80x9cspreadxe2x80x9d user information signals over a predetermined spectral band prior to modulation onto a carrier signal for transmission as communication signals. PN spreading is a method of spread-spectrum transmission that is well known in the art, and produces a communication signal with a bandwidth much greater than that of the data signal. In the base station- or gateway-to-user communication link, PN spreading codes or binary sequences are used to discriminate between signals transmitted by different base stations or over different beams, as well as between multipath signals. These codes are typically shared by all communication signals within a given cell or beam, that are on a common frequency (sub-beam).
In a typical CDMA spread-spectrum communication system, channelizing codes are used to discriminate between different users within a cell or between user signals transmitted within a satellite sub-beam on a forward link (i.e., the signal path from the base station or gateway to the user transceiver). That is, each user transceiver has its own orthogonal channel provided on the forward link by using a unique xe2x80x98channelizingxe2x80x99 orthogonal code. Walsh functions are generally used to implement the channelizing codes.
Wide band CDMA techniques permit problems such as multipath fading to be more readily overcome and provide a relatively high signal gain. However, some form of signal diversity is also generally provided to further reduce the deleterious effects of fading and additional problems associated with acquiring and demodulating signals in the presence of relative user, or repeater, movement, which along with large distances causes substantial dynamic changes in path lengths.
Generally, three types of diversity are used in spread spectrum communication systems, including time, frequency, and space diversity. Time diversity is obtainable using repetition and time interleaving of signal components, and a form of frequency diversity is inherently provided by spreading the signal energy over a wide bandwidth.
Space or path diversity is obtained by providing multiple signal paths through simultaneous links with a user through two or more base stations or antennas, for terrestrial-based repeater systems; or two or more satellites or satellite beams, for space-based repeater systems. That is, for terrestrial systems signals can be transferred through multiple base stations, or more likely, through multiple antennas servicing various cell sectors. For satellite communication systems, path diversity is typically obtained by transferring signals over multiple paths using either multiple satellites (repeaters) or multiple transponder beams on a single satellite. However as discussed below, the latter approach is not generally useful.
Examples of using path diversity in multiple access communication systems are illustrated in U.S. Pat. No. 5,101,501 entitled xe2x80x9cSoft Handoff In A CDMA Cellular Telephone System,xe2x80x9d issued Mar. 31, 1992, and U.S. Pat. No. 5,109,390 entitled xe2x80x9cDiversity Receiver In A CDMA Cellular Telephone System,xe2x80x9d issued Apr. 28, 1992, both assigned to the assignee of the present invention, and incorporated herein by reference.
Typical spread spectrum communication systems also contemplate the use of a xe2x80x9cpilotxe2x80x9d carrier signal as a coherent phase reference for gateway- or satellite-to-user and base station-to-user links. That is, a pilot signal, which typically contains no data modulation, is transmitted by a base station or gateway throughout a given region of coverage. A single pilot is typically transmitted by each gateway or base station for each frequency used, typically referred to as a CDMA channel, or sub-beam. This pilot is shared by all user terminals receiving signals from that source. This provides signals that can be readily distinguished from each other, also distinguishing between beams and cells while providing simplified acquisition and tracking.
Pilot signals are used by subscriber units to obtain initial system synchronization, and provide robust time, frequency, and phase tracking of transmitted signals. Phase information obtained from tracking a pilot signal carrier is used as a carrier phase reference for coherent demodulation of communication system or user information signals.
Pilot signals are also generally used to gauge relative signal or beam strength for received communication signals. In many systems, pilot signals are also generally transmitted at a higher power level than typical traffic or other data signals to provide a greater signal-to-noise ratio and interference margin. This higher power level also enables an initial acquisition search for a pilot signal to be accomplished at high speed while providing for very accurate tracking of the pilot carrier phase using relatively wide bandwidth, and lower cost, phase tracking circuits.
As satellites transit in their respective orbits, the beams they project onto the Earth move relative to users, periodically changing which satellites can provide service for particular users. This occurs for example as satellites come into or disappear from xe2x80x9cviewxe2x80x9d. The same effect also occurs between beams in a single satellite, with service for particular users changing as the beams move across the earth""s surface. In addition, mobile users sometimes move relative to beams or satellite paths, also causing beam coverage or service areas to change. In these situations, communication links for signals must be handed off between beams. A similar process occurs for terrestrial cellular systems where users move relative to base stations and sectors or sector boundaries within cells.
A basic technique developed to prevent loss of signal and improved transfer of information is the so-called xe2x80x9csoftxe2x80x9d handoff scheme which is described in U.S. Pat. No. 5,101,501, referred to above. In this technique, a new link or signal path is established through a new satellite, or satellite beam, before the existing or old link is disconnected or discarded. The information (energy) available for a given communication signal from each path can be combined to provide improved signal reception, as well as prevent disconnected communication links. This can be done for either the forward link communications from gateway-to-user terminal, or the reverse link communications from user terminal-to-gateway. For the reverse link, the diversity combining process is accomplished at the gateway or within a centralized control or switching center.
Unfortunately, when using soft handoff techniques in satellite communication systems several problems arise. While diversity can be used to improve signal characteristics for communication links involving multiple satellites, it is not useful for communicating to a user through multiple beams on a single satellite. Beams from a single satellite have virtually the same path at the same frequency on a forward link, with nearly the same transit time, and have the same fading or interference characteristics. Diversity combining two such forward link signals provides little benefit, while unnecessarily consuming power and adding to the background noise level or interference.
Users can also traverse between adjacent beams quickly and move back and forth along their respective boundaries. If a user is moving along the Earth""s surface perpendicular to the direction of sweep for a satellite spot containing a series of beams, the user might traverse between two adjacent beams repeatedly. In this situation, a user can switch between adjacent beams on a frequent basis, especially where the beams are near the edge of coverage for a satellite spot. In addition, other factors such as low satellite elevation and local terrain or signal blockage continuously impact signal quality. In this situation, the communication system may be continuously switching between beams in a soft handoff mode to maintain a best communication link.
A similar process may occur for mobile users moving around in sectored cells in terrestrial communication systems. That is, where the cells are subdivided into two or more smaller service areas which are covered at differing frequencies or using different code spaces. Here, mobile users may travel along or repeatedly cross sector boundaries within a cell, depending on such factors as cell and sector size and local physical environment. The resulting switching activity may be increased by the use of techniques meant to otherwise increase cell capacity. For example, a cell may employ a series of relatively small sectors or sectors having adjustable sizes to increase capacity or accommodate certain traffic patterns relative to the cell service area. However, smaller sectors and more sector boundaries increase the likelihood of more frequent handoffs between sectors. Changing sector sizes may also shift a user terminal back and forth between adjacent sectors with a minimum amount of physical movement.
This switching activity tends to consume excessive system resources in several ways. First, the time spent establishing links and selecting channels, with corresponding signal time, frequency, and phase tracking, error detection, and so forth, consumes signal processing resources which could be applied to other tasks such as signal demodulation, diversity combining, and decoding. Second, for a substantial period of time, multiple orthogonal channels in each beam are in use by a single user. That is, orthogonal codes in adjacent beams, or sectors, are allocated to a single user. Since there are a relatively limited number of such orthogonal channels available in the communication system, this decreases effective system capacity. Third, additional power is consumed maintaining each active channel for a single user, double for two channels, and energy deposited into such communication channels causes interference, which is deleterious to system operation.
Therefore, what is needed is a handoff technique which allows a soft handoff between adjacent beams from a single satellite with decreased system resources when the user is traversing between such beams. The technique should also address soft handoff between adjacent sectors within a cell serviced by a base station or cell-site. The method should provide a solution that decreases unnecessary consumption of system resources while remaining compatible with other soft handoff schemes.
In view of the above problems encountered in the art, one purpose of the present invention is to provide a technique for handing off or transferring communication links between adjacent service areas defined by beams of a single satellite or sectors in a cell, while minimizing utilization of system resources.
An advantage of the present invention is that soft handoff can be employed for reverse link signal transfer while being eliminated or used less frequently and/or for shorter durations on forward link transfers.
Another purpose of the invention is to reduce switching and communication signal tracking and control operations during transfers between adjacent service areas for single satellites and cells.
Another advantage of the invention is that system capacity can be increased by increasing the general availability of orthogonal channelizing codes and traffic channels.
Yet another advantage of the invention is that certain pilot signal adjustments can be accommodated more accurately, allowing increased system capacity.
These and other purposes, advantages, and objects of the present invention are realized in a method and apparatus for performing handoff between adjacent service areas in a wireless communication system that transfers communication signals using at least one central communications station which establishes geographical service areas for user terminals operating within the system. The central station is generally either a gateway that establishes adjacent service areas using satellite beams from a single satellite, or a single base station that establishes adjacent service areas as sectors of a cell.
A physical transition of a user terminal between two adjacent service areas, each established by a common central communications station, is detected by determining the signal strength for signals originating from the adjacent service areas. While the user terminal continues to use a forward link channel in a first service area, the use of a forward link channel in a second service area is set up. This action is taken when a detected signal strength for the second adjacent service area at least equals that of the first service area. Once the forward link traffic channel is established in the second service area, its satisfactory operation is confirmed according to a preselected minimum quality level, based on various known criteria, and the forward link for the first service area is disengaged or inactivated. Applicable criteria are based on known factors, such as on determining if the new channel has sufficient energy, or a sufficiently low error rate to maintain a desired level of communication service.
Preferably either pilot or paging signals associated with the service areas form the signals used for detecting service area transitions, and the strength of such signals determines a signal strength for each service area relative to the user terminal position. The pilot or paging signals are received using at least one user terminal receiver, and their strength is measured using known techniques and processing elements. The strength of signals from different service areas can then be compared, typically by at least temporarily storing one or more measurements for operation on by one or more comparators, control processors, or other known processing elements.
Preferably, signal strength measurement information is transmitted as part of one of several known types of signals to the central station, which receives the measurement information using known signal reception means and techniques. The central station then compares received signal strength values and determines relative signal strengths. The central station may use additional signal information available internally as part of this comparison or in determining signal strength.
The central station can then use a communications transmitter to transmit the results of this comparison to the user terminal. At the same time, the central station can set up a desired new channel through the new service area to be used, in accordance with known capacity limitations, or various channel assignment procedures and schemes. By periodically reporting pilot signal measurements to the central station, a need for new channels can be more readily anticipated, allowing some channels to even be reserved as desired.
Alternatively, the signal strength measurement information is used by the user terminal to detect and compare the signal levels for the two adjacent service areas. The user terminal determines that a transition between the service areas is occurring, or that the relative strength of a signal from a new service area exceeds that currently in use. The user terminal sends this information to the gateway or base station, instead of sending signal measurement information. The gateway again determines if a new traffic channel can be assigned, and assigns the new channel, as appropriate to implement the handoff.
In further aspects of the invention, the presence of adjusted pilot signals is detected. That is, a means is used to detect pilot signals being received that have had their power adjusted during transmission to boost signal strength and compensate for signal roll-off conditions near the edges of beams. When such adjusted pilot signals are detected, a so-called a compensation factor is derived for each one which has substantially the same magnitude as the boost or increase applied to the signal. This compensation factor is, then applied as a negative adjustment or bias to the signal level during the strength measurements for each such adjusted pilot signal to compensate for the artificial boost in power and arrive at a more accurate non-adjusted strength determination. This compensation factor or value can be applied either at the user terminal or the central station, as desired.
In addition, the central station can synchronize the timing of communication signals and forward link channels for a user terminal through both old and new service areas. This can be done when either the gateway or the user terminal determines that a new forward link channel is desired for the user terminal in a new service area. By using appropriate signal timing and control elements in the central station, the signal timing can be synchronized so that the forward link of the first service area can be disengaged and the use of the forward link channel for said second service area commenced at substantially the same time.
It is very desirable to prevent undue switching between beams and a corresponding expenditure of system resources. Therefore, in further aspects of the invention, a form of hysteresis can be used in which the value for at least one pre-selected communication parameter is inspected on a periodic basis. Any request for a new forward link channel is either prevented from being generated or blocked from transfer until a minimum change in the monitored value has occurred, since a new forward link channel was previously requested. Exemplary parameters are time and signal energy level. The user terminal can determine when a pre-selected minimum period of time has passed since a new forward link channel was previously requested, or when a pre-selected minimum signal level has been reached by a current service area signal before requesting a forward link channel.
This can be implemented, for example, by storing signal identification information for each service area used, up to a predetermined maximum number, in a memory for a predetermined maximum length of time. Signal identification for any newly detected service area is then compared to stored identification information to determine if the same service area is being detected again, and within a restricted period of time. This information can be used by central stations, gateways or base stations, to limit the amount of inter-beam or inter-sector switching.